Six hundred million years ago, a meteorite smashed into the Lake Acraman region of southern Australia at more than 1,000 kilometers per second, sending shock waves through the surrounding volcanic rock and creating concentric impact ridges up to 160 kilometers across. The impact sprayed rock and dust as far as 300 kilometers away; the debris settled in the quiet sea nearby. Plentiful marine sediments quickly covered the debris, preserving the story of the meteorite's impact from erosion.

Such is the scene depicted in the July 11 SCIENCE by a group of researchers from the University of Adelaide (Australia), the Australian National University in Canberra and the University of Arizona in Tucson. Their discovery, made while they were trying to date volcanic rocks in the region, marks the first time a meteorite impact has been linked directly to its debris and is the most complete record of an impact prior to the Cenozoic era, which began 70 million years ago.

Some geologists have suggested that craters such as the one at Lake Acraman could have formed through volcanic eruptions, which would also spray debris over the surrounding terrain. The investigators at the Lake Acraman site, however, present several arguments in favor of the meteorite theory for the crater in southern Australia.

The volcanic material found among the debris, the investigators report, closely resembles, and probably originated in, volcanic rock from the Gawler Range, the site of the crater. However, the volcanic debris is more than twice as old as the sediments in which it was deposited, indicating it already existed at the time the crater was formed. Moreover, the volcanic fragments do not show the characteristic bomblike shape resulting from an eruption, but do show the cone-shaped fracture lines characteristic of a high-velocity shock that, the researchers write in their paper, only a meteorite could produce.

A complete picture of a meteorite impact could shed light on the role of meteorites in mass extinctions, a topic of continuing debate. "Maybe it can tell us how big a dust cloud [the meteorite] might have created," says geologist Gerta Keller of Princeton (N.J.) University, referring to the idea advanced by Walter Alvarez of the University of California at Berkeley that debris blown into the atmosphere by meteorite impacts periodically cooled the earth and extinguished much of the life on the planet (SN:6/2/79,p.356).

Astronomers can also use the information to refine their estimates of the ages of surface structures on other planets, according to Richard Grieve of the Geological Survey of Canada in Ottawa. By knowing the number and ages of craters on earth, scientists can calculate the frequency of meteorite impacts for specific time intervals, then count the craters on another planet to estimate the age of that planet's surface.

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